1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor memory device, and more particularly, to the improvement of the isolation characteristics of a dynamic random access memory (DRAM) device.
2. Description of the Related Art
Recently, as DRAM devices have become more fine-structured, openings such as contact holes have also become more fine-structured. On the other hand, in order to increase the integration, as technology for forming a field insulating layer, a shallow trench isolation (STI) process has been adopted instead of a local oxidation of silicon (LOCOS) process.
In a prior art method for manufacturing a DRAM device, a plurality of openings are perforated in an insulating layer formed on first impurity diffusion regions for bit lines and second impurity diffusion regions for capacitors surrounded by a field insulating layer of a semiconductor substrate, and each of the openings corresponds to one of the impurity diffusion regions. This will be explained later in detail.
In the above-described prior art method, since each opening corresponds to one of the impurity diffusion regions, the sides of the field insulating layer may be etched, which would deteriorate the isolation characteristics. Particularly, if a field insulating layer is formed by an STI process so that the sides of the field insulating layer are sharp, the isolation characteristics would remarkably deteriorate. Also, since the size of contact structures, i.e., pad electrodes buried in the openings is decreased, the contact resistance is increased.